Heat treatment processing process

Heating and cooling are indispensable steps in the process of heat treatment

Heating is one of the important processes of heat treatment. Metal heat treatment heating methods are many, the first use of charcoal and coal as heat sources, and then the use of liquid and gas fuel. The application of electricity makes heating easy to control without environmental pollution. These heat sources can be directly heated or indirectly heated by molten salt or metal or even floating particles.

When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization (that is, the carbon content on the surface of steel parts decreases) often occur, which has a very adverse effect on the surface performance of the parts after heat treatment. Therefore, metal should be heated in controlled atmosphere or protective atmosphere, in molten salt and in vacuum.

Heating temperature is one of the important technological parameters of heat treatment process. The selection and control of heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies according to the metal being treated and the purpose of the heat treatment, but is generally heated above the phase transition temperature to obtain high-temperature microstructure. In addition, the transformation needs a certain time, so when the surface of the metal workpiece reaches the required heating temperature, it must be kept at this temperature for a certain time, so that the internal and external temperature is the same, so that the microstructure changes completely, this period of time is called the holding time. When high energy density heating and surface heat treatment are adopted, the heating speed is extremely fast, so there is usually no holding time, while the holding time of chemical heat treatment is often longer.

Cooling is also an indispensable step in the process of heat treatment. In general, the cooling rate of annealing is the slowest, the cooling rate of normalizing is faster, and the cooling rate of quenching is faster. However, there are also different requirements for different types of steel, such as hollow steel can be quenched with normalized cooling rate.

Metal heat treatment processing technology can be divided into general heat treatment, surface heat treatment and chemical heat treatment three categories. According to the difference of heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. With different heat treatment processes, different tissues can be obtained for the same metal, which has different properties. Steel is the most widely used metal in industry, and the microstructure of steel is the most complex, so there are many kinds of heat treatment processes.

Integral heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate rate to change its overall mechanical properties. There are four basic processes for the heat treatment of steel: annealing, normalizing, quenching and tempering.